The invention relates generally to honeycomb substrates and particularly to a method of enhancing the isostatic strength of a honeycomb substrate having cells defined by thin webs.
Honeycomb substrates are widely used as catalyst carriers for purification of exhaust gases from internal combustion engines. A typical honeycomb substrate has a columnar body, the cross-sectional shape of which is typically round or oval. An array of parallel, straight channels is formed in the columnar body to allow passage of gases through the honeycomb substrate. The channels run axially along the length of the columnar body. The cross-section of the channels can be of any arbitrary shape, such as triangle, square, and hexagon. For automotive applications, the cross-section of the channels is frequently square because square channels are easier to manufacture. The channel walls may be coated with a porous washcoat containing an active catalyst. Alternatively, the active catalyst may be incorporated directly into the channel walls. In operation, exhaust gases flow into the channels and are converted into less noxious components in the presence of the active catalyst prior to exiting the honeycomb substrate. For illustration purposes, FIG. 1 shows a cross-section of a typical prior-art honeycomb substrate 100 having an array of square cells (or channels) 102 defined by an array of square webs (or cell walls) 104 and bounded by a skin (or peripheral wall) 106. Typically, the skin 106 has a circular or elliptical profile.
A honeycomb substrate is typically wrapped in a mat and inserted in a metal can prior to use in an automotive application. When the honeycomb substrate is inserted in a can, the forces required to restrain the honeycomb substrate within the can are uniformly distributed along the periphery of the honeycomb substrate, perpendicular to the skin of the honeycomb substrate. These forces have the greatest impact for the honeycomb substrate with square cells when applied at the 45° positions to the square webs, i.e., when applied in a direction along the diagonals of the square webs, as shown by the arrows in FIG. 1. When loaded at this angle, the webs cannot function as columns under compression, and the honeycomb substrate is less rigid. In this state, the webs are subjected to high deflections, which generate bending moments and undesirable tensile stresses in the honeycomb substrate. Typically, the honeycomb substrate is made of ceramic, a material that is inherently weak in tension. Hence, the tensile stress levels determine the strength of the canned ceramic honeycomb substrate.
In an effort to meet stringent automotive emission requirements, it has been necessary to reduce the thermal mass in the central region of the honeycomb substrate while increasing the geometric surface area and open frontal area of the honeycomb substrate. The thermal mass in the central region has been reduced and the geometric surface area has been increased by increasing the number of cells in the honeycomb substrate. At the same time, the thickness of the webs in the honeycomb substrate has been significantly reduced to increase the open frontal area of the honeycomb substrate and limit the back-pressure in the honeycomb substrate to an acceptable limit. Although reduction in web thickness has improved emissions performance, it has also resulted in marked reduction in the isostatic strength of the honeycomb substrate, making the honeycomb substrate more susceptible to damage during canning and lowering the thermal and mechanical durability of the honeycomb substrate in the application.
Various methods have been proposed for improving the isostatic strength of ceramic honeycomb substrates with thin webs. Some of these methods include increasing the cell density at a region near the periphery of the honeycomb substrate, thickening the webs at a region near the periphery of the honeycomb substrate, and using additives to strengthen the skin of the honeycomb substrate. One approach to increasing the cell density near the periphery of the honeycomb substrate involves subdividing the square cells near the periphery of the honeycomb substrate into smaller square cells or triangle cells. One method for increasing the thickness of the webs near the periphery of the honeycomb substrate includes rounding the square cells near the periphery of the honeycomb substrate. While these approaches can improve the isostatic strength of the honeycomb substrate, they can also yield undesirable results such as increase in pressure loss across the honeycomb substrate and/or reduction in thermal shock strength of the honeycomb substrate.
From the foregoing, there is desired a method of increasing the isostatic strength of a honeycomb substrate in all directions without adversely impacting the performance of the honeycomb substrate.